The overall goal of this proposal remains to investigate how sciatic ligation produces plastic changes in dorsal horn neuronal excitability to ultimately contribute to the development of neuropathic pain. In the past funding period we sought to establish whether electrophysiological correlates of synaptic plasticity could be recorded in the spinal dorsal horn in vivo. We also wanted to determine whether these phenomena were a normal feature of spinal nociceptive processing, or were only elicited after injury. We established that in control animals, a given tetanic protocol favored post-tetanic depression, while in ligated animals the same tetanic protocol favored potentiation. We assume that both intracellular and extraneous (synaptic) influence combine to produce a given post-tetanic response, and that a balance between excitatory and inhibitory processes determines whether potentiation or depression will ensue. In this competitive renewal we wish to: (1) determine which processes contribute to the balance between potentiation and depression, and (2) establish how sciatic ligation affects these processes. Overall, we hypothesize that a critical change in the action of one or more of these processes leads to the differential pattern of post-tetanic responses between control and ligated rats. In Specific Aim 1 we will continue to record sciatic-evoked dorsal horn A beta and A delta fiber field potentials. We will employ selected agonists, antagonists, activators or inhibitors to test two specific hypotheses as we ask: (1) Does GABAA receptor-mediated inhibition contribute to the depression in control rats, and does the loss of this inhibition contribute to the potentiation in ligated animals? (2) Does activation of calcineurin contribute to the depression in control animals, and is the loss of this action reflected in the ligated response? Similarly, does protein kinase A (PKA) activation contribute to the initial potentiation in both groups of animals, or is the kinase activated only in ligated animals? Additional preliminary data suggest that metaplasticity may play a critical role in spinal nociception. In this Aim we will also examine the roles of GABA, PKA and calcineurin in spinal metaplasticity. In Specific Aim 2 we will focus on the cAMP responsive element binding protein (CREB). Recent evidence suggests that phosphorylation of CREB plays a critical role in long-lasting synaptic plasticity. We will employ immunocytochemical and Wesern immunolot procedures to test two specific hypotheses as we ask: (1) When does pCREB (pCREB) first appear in ligated rats, and does its appearance and disappearance correlate with thermal hyperalgesia and synaptic plasticity? and (2 Do changes in immunoreactive PKA and calcineurin content in the spinal dorsal horn parallel the onset and disappearance of thermal hyperalgesia and spinal synaptic plasticity?